1. Field of the Invention
The present invention generally relates to wireless communications, and more particularly to an apparatus and method for determining when to trigger and transmit sounding packets used to characterize the channel in a transmit beamforming system.
2. Description of the Related Art
Transmit beamforming (TxBF) technology entails using multiple, separate antennas to transmit single or multiple signal streams to a receiver. The raw data signals are properly weighted and combined for each of the transmitter's antennas so that a transmit beam is formed and directed to the receiver. The intent is to have all of the transmitted raw data signals from the multiple antennas coherently combine at the receiver. By tightly focusing the wireless data signals at the receiver, the transmitter's range is effectively extended and higher data rates are made possible.
In practice, the wireless data signals can encounter a variety of scatters. Sources of scattering are unpredictable and variable in nature. For example, objects (e.g. buildings, walls, furniture, fixtures, etc.) in the way of a wireless data signal may cause the wireless data signal to be reflected in multiple, different directions. As a result, the original transmitted wireless data signal may ultimately reach the receiving antenna by way of two or more different paths. This propagation phenomenon is referred to as “multipathing.” Conventionally, scattering is treated as a problem because a single transmitted wireless signal can exhibit different delays (e.g. phases) and magnitudes as experienced by the receiver, depending on the different paths that were traveled.
Although scattering and multipathing cannot be eliminated, their effects can be measured and then factored into the calculations when determining the beamforming matrix. For example, a specific channel's information can be measured. Based on that channel's information, the transmitter can pre-process the raw data signals so as to utilize the specific channel characteristics. One method for determining the channel characteristics is set forth by the Institute of Electrical and Electronics Engineers (IEEE) in 802.11n. In 802.11n, a set of standards is defined for wireless networking (e.g. Wi-Fi) that is widely adopted by the wireless communications industry. In 802.11n, sounding packets are transmitted to the receiver. The sounding packets contain training symbols that, upon receipt by the receiver, are interpreted to characterize the channel. Based on the calculated channel characteristics, a beamforming matrix or vector can be applied to weight and combine the raw data signals. Using the beamforming matrix, the wireless data signals can be coherently combined at the receiver, even in the presence of scattering and multipathing.
Under some operating conditions, the environment is fairly static (e.g. indoors). In these cases, the originally calculated channel characteristics are slowly varying over time. Therefore, these channel characteristics can be used over relatively long periods of time with minor or negligible performance degradation. Consequently, the times between re-transmitting sounding packets to update the channel characteristics can be extended.
However, there may be operating conditions in which the environment is rapidly changing, i.e. the channel characteristics significantly changing from one moment to the next. In these circumstances, sounding packets should be more frequently re-transmitted to ensure accurate, up-to-date channel characteristics. Otherwise, the performance may suffer to the point of losing the benefits of TxBF altogether.
Unfortunately, transmitting sounding packets consume valuable bandwidth. Specifically, user data cannot be sent while sounding packets, sounding long training fields (LTFs), or feedbacks to sounding packets (e.g. CSI/V/CV, described below) are being transmitted.
As a result, TxBF designers are faced with a dilemma. On the one hand, transmitting fewer sounding packets reserves airtime to be better utilized for the transmission of user data. The downside is that performance may suffer, especially in a rapidly changing environment. On the other hand, the frequent transmissions of sounding packets lead to improved performance. However, frequent transmissions of sounding packets consume valuable bandwidth. Further complicating matters is that an otherwise stable operating environment may occasionally undergo abrupt and rapid changes. Conversely, a rapidly changing environment may encounter periods of stability. Moreover, the TxBF system could be moved from a stable environment to a rapidly changing environment or vice versa. Thus, a need arises for an improved TxBF technique that efficiently triggers sounding in a changing environment.